Handover in Scale of the Fluxes of Momentum, Heat, etc. in the Atmospheric Boundary Layer

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Abstract
The handover of the vertical fluxes of momentum, heat, etc., from the small‐scale turbulence close to the earth's surface into the larger‐scale motions above is becoming an important question in the context of numerical treatment of atmospheric dynamics. In convective conditions, a decoupling of heat flow from the surface value is suggested at a height of some 5 to 10 times the Obukhov scale height, at times lower. Some changes in regime are already well documented below this level. Above it, the need for direct measurement of the eddy heat flux on the cumulus scale, and location of the roots of these circulations, is emphasized. For momentum flux the handover from microscale direct into the synoptic scale is considered primarily. At a level where the microscale flux has fallen from its surface value τ0 to τ, the vertical motion in unaccelerated flow is ρw = curl [(τ0 − τ)/f], where f is the Coriolis parameter. When this is viewed as a component of eddy motion on the global scale, the resultant eddy transport of momentum is found to be insignificant. But the asymmetry of acceleration patterns in the westerlies, particularly that broadly associated with the transient frontal systems, appears to yield significant momentum flux in the required direction. This defines the need for a clearer identification of the typical acceleration patterns and for numerical solutions of the appropriately modified boundary layer equations. The momentum flux attributable to thermal wind and the handover into mean meridional circulation are also discussed.

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